1. Technical Field of the Invention
The present invention relates to a magnetic recording medium comprising a nonmagnetic support provided with a magnetic layer directly thereon or via a nonmagnetic layer, and having excellent running durability and electromagnetic conversion characteristics.
2. Description of the Related Art
In the field of magnetic tape, with the prevalence of office computers, such as minicomputers, personal computers and work station, magnetic tapes for recording computer data as external storage media (so-called backup tapes) have been vigorously studied in recent years. For putting magnetic tapes in such uses to practical use, the improvement of recording capacity has been strongly demanded conjointly with the miniaturization of a computer and the increase of information processing performance for achieving high capacity recording and miniaturization.
Magnetic recording media comprising a nonmagnetic support having coated thereon a magnetic layer comprising an iron oxide, a Co-modified iron oxide, CrO2, ferromagnetic metal powder (MP), or hexagonal ferrite powder dispersed in a binder have been conventionally widely used. Of these powders, ferromagnetic metal fine powders and hexagonal ferrite fine powders are known to be excellent in high density recording characteristics.
Magnetic heads working with electromagnetic induction as the principle of operation (an induction type magnetic head) are conventionally used and spread. However, the magnetic heads of this type are approaching their limit for use in the field of higher density recording and reproduction. That is, it is necessary to increase the number of winding of the coil of a reproduction head to obtain larger reproduction output, but when the winding number is increased, the inductance increases and the resistance at high frequency heightens. As a result, the reproduction output lowers. In recent years, reproduction heads that work with magneto-resistance (MR) as the principle of operation are proposed and get to be used in hard discs and the like. The application of the MR head to magnetic tapes is proposed in JP-A-8-227517 (The term “JP-A” as used herein refers to an “unexamined published Japanese patent application”.). As compared with the induction type magnetic head, several times of reproduction output can be obtained by using MR head. Further, since an induction coil is not used in MR head, noises coming from instruments, e.g., impedance noises, are greatly reduced, and it has become possible to obtain a great S/N ratio or C/N ratio by lowering the noise coming from magnetic recording media. In other words, good recording and reproduction can be done and high density recording characteristics can be drastically improved by lessening the noise of magnetic recording media hiding behind the instrument noises. Further, it is required of magnetic recording media, in particular, backup tapes for computers, to be excellent in durability and free from defects of data. For securing such excellent electromagnetic conversion characteristics and durability of magnetic recording media, the increase of coercive force (Hc) and orientation property of magnetic powders, the development of protective films of magnetic layers, and the development of lubricants to reduce the friction coefficient between magnetic layers and back layers have been performed. On the other hand, on the side of magnetic recording and reproducing apparatus, as the means for increasing recording capacity per a unit area, shortening of wavelength of recording frequency and narrowing of the track width of a magnetic recording head are advanced. For instance, in cartridge type recording media, it has been tried to increase the capacity by loading a longer tape by thinning the thickness of the tape while maintaining the capacity of a cartridge as it is. A typical example is the increase of the capacity of from DDS2 system to DDS3 system of a backup tape for computer (Report on Research of the Trends of the Production and Demand of Recording Media in the World and Technical Tendency, P97, published by Nippon Recording Media Industry Association). Further, the improvement of area recording density has been advanced year by year by narrowing the track width of recording and reproducing heads. In such a system, the control of positioning of recording and reproducing heads and a magnetic recording medium is important. In a tape medium, since more stable running is necessary when a tape runs in a recording/reproducing apparatus, the accuracy of the position of a tape running guide and the position of the flange for regulating a tape is important. However, falling of a magnetic layer, a backing layer and a support from the tape edge occurs when the positioning regulation is too strict. As for the durability of a magnetic layer surface, binders having high durability and lubricants for reducing a friction coefficient are developed, and DLT that is now the mainstream of the backup tape for computer having a tape running speed of 2.5 m/s has been commercialized without generating problems in durability of magnetic layers. However, the influence on error rate by the adhesion of the debris of a magnetic layer, a backing layer and a support to the tape due to falling from the tape edge has been actualized. LTO commercialized in recent years has a tape speed as fast as 5 m/sec, and the problem of adhesion of the debris of a tape edge (edge debris) to a tape and a head has now become a great concern.
For the purpose of preventing a pancake-shaped failure by suppressing high edge of the edge part occurring in a slit process, a magnetic recording medium comprising a polyethylene naphthalate support having a thickness of 4 μm or more, wherein the ratio of Young's modulus of the support in the machine direction to that in the transverse direction is prescribed to be from 0.4 to 1.5, and a viscosity to be from 0.45 to 0.53 is disclosed in JP-A-8-45060.
However, as the latest support of a magnetic recording medium improved in recording density, a sufficient support cannot be ensured only with the above prescription. Regarding the viscosity disclosed in JP-A-8-45060, the unit and the measuring method are not disclosed, so that the invention is unclear. With respect to the material of the support, the description is only concerning polyethylene naphthalate and there is no description of using fillers.
As described above, conventional supports cannot provide a magnetic recording medium corresponding to the requirement of the latest high density recording.